Various methods and apparatus for packaging and shipping goods have heretofore been proposed and implemented. For example, multiple unit products can be stacked in boxes for shipment. Furthermore various boxes or articles may be unitized in a pallet load having a rectangular, square or irregular cross section for shipment by wrapping various boxes or articles with a web of stretched plastic film.
Such arrangements generally include a mechanism which dispenses a web of plastic film as it rotates about the pallet load. Such plastic film may be prestretched between rollers prior to being wrapped around the pallet load.
Generally speaking, the plastic film which is dispensed about said pallet load encounters different load demands or tension as it rotates about the pallet load edges. More particularly, as the plastic film rounds the corner or edge of a pallet load the plastic film must be supplied at a faster speed in order to maintain a constant tension in the plastic film. Various apparatus and method have heretofore been proposed to attempt to maintain a relativiely constant tension in the plastic film.
For example, U.S. Pat. No. 4,676,048 is directed towards apparatus and process for applying stretchable plastic film to loads using a prestretch mechanism which is driven and controlled independently of the web tension to minimize the variation in the force exerted on the film web at load edges by varying the film web supply speed.
Moreover, U.S. Pat. No. 4,628,667 discloses a wrapping machine for wrapping a load in a unitary packaging, said wrapping machine including a mechanism controlling the operation of a variable speed DC driving motor so as to rotate a support at a first speed during a first portion of the rotatable movement of the support and then at a second slower speed during a subsequent portion of the rotatable movement of the support so as to gradually stop the rotation of the support and avoid damage to either the load or stretchable material being used for wrapping the load.
Moreover, U.S. Pat. No. 4,524,568 teaches a rotatable film wrapping apparatus for wrapping a load on a conveyor with an overwrap of stretched plastic film web having a film web prestretching mechanism which is provided with a constant torque by a power assistance mechanism so that the force on the film to the load can be substantially reduced while allowing the prestretch drive to speed up and slow down according to the film demand from corner or changing load.
Furthermore U.S. Pat. No. 4,497,159 discloses apparatus for applying stretchable plastic film to loads using two frictionally connected rollers which are held together and driven by the film web at different speeds to elongate plastic film beyond its yield point.
Also U.S. Pat. No. 4,590,746 discloses a tension maintaining device which applies a biasing force against a tension roller over which stretchable material passes so as to maintain a constant tension on the stretchable material.
Each of the prior art devices generally disclose complicated structure. Moreover, such prior art devices have a relatively slow response time wherein there is a lag betwen the time when a greater supply of plastic web is required and the time that it is actually supplied so that in many cases the plastic film breaks due to the increased tension imparted thereon.
It is an object of this invetion to provide apparatus and a method for dispensing plastic film which is simple to construct and capable of maintaining a preselected tension in the film web as it rotates about the pallet load.